In traditional robotic applications, the robotic arms are of stiff structures so as to enable achieving the highest possible positioning accuracy. Each of the segments of such robotic arms is of large weight and is hard to backdrive, which is not a problem in some of the tasks—e.g. welding or placing—to be carried out. Robots are used for performing such tasks as well, where instead of position-controlling the robotic arm, force-controlling is needed. Such tasks generally comprise interaction with the environment, solved by means of external sensors. End-points of the robotic arms are equipped with a force gauge or torque gauge, so as to enable regulation of the interaction forces acting between the robotic arm and the environment. Force feedback control through feedback is most widely used in robotics.
It is a disadvantage of these solutions, however, that the thereby achieved active control is characterized by a low bandwidth. By that, it is meant that a limited reaction can only be exerted as a response to a sudden change in the force caused by, for instance, a collision. Low bandwidth is due to the latency, i.e. low speed of the force gauge sensors only to a smaller degree, but it is caused mainly by the mechanism of the robotic joints. Known robotic joints are namely characterized by using relatively large transmission ratio so as to achieve the highest possible torque. Consequently, these have a high moment of inertia, and therefore can only react to sudden movement in a restricted measure. The mechanism of robotic joints is hereinafter also referred to as actuators.
This problem represents a significant problem in various fields of robotics. With regard to walking robots, by way of example, in case of the repetitive landings, which means high impact incidents when running or jumping, the robots are unable to react in an appropriate manner due to the aforementioned disadvantages. This problem is in many cases repaired by the insertion of elastic members; one such solution is disclosed in the following study: R. V. Ham, B. Vanderborght, M. V. Damme, B. Verrelst, D. Lefeber, MACCEPA, the mechanically adjustable compliance and controllable equilibrium position actuator: Design and implementation in a biped robot, Robotics and Autonomous Systems, Vol. 55, pp. 761-768, (2007). A further example for such a solution is SEA (Series Elastic Actuator) disclosed in patent document U.S. Pat. No. 5,650,704, wherein, generally, a spring is inserted after an electric motor having a transmission of high transmission ratio. This solution may ensure an increased shock-tolerance despite the high moment of inertia caused by the large transmission ratio of the motor. Additionally, the solution enables measurement of the respective torques acting upon the individual actuators in view of the relationship between elongation and torque of the elastic member by means of direct measurement of the deformation of the inserted spring. It is possible to realize force-control on the level of the individual joints with this solution, thereby rendering the system safer. At present, this is the most advanced force-control system being prevalent in this field, but it has several disadvantages, mainly due to the insertion of the elastic member, i.e. the mechanic realization of the elastic behavior.
To solve certain robotic tasks the control of the mechanical impedance is necessary. By way of example, let us refer to robotic arms and legs, wherein structural compliance, namely the force acting against backdrive is to be controlled for the appropriate behavior, i.e. the structure is to have variable stiffness, elasticity. Accordingly, variable elasticity of the adjoining points is to be enabled. To achieve this, in many cases a number of elastic members are inserted in various ways. The known implementations are summed up in the following study: R. van Ham, Th. G. Sugar, B. Vanderborght, K. W. Hollander, and D. Lefeber, Compliant actuator designs, IEEE Robotics & Automation Magazine, Vol. 16, No. 3 (2009), pp. 81-94.                Equilibrium-controlled stiffness: An example for this solution is the aforementioned SEA, wherein virtual stiffness is established by an active control hiding the original parameters of the built-in spring. By measuring the displacement of the joint, by repositioning the non-elongated state of the physical spring, i.e. by displacing its respective end-points, is the desired stiffness—spring constant—attempted to be set. A major disadvantage of this solution is that the variable stiffness is created by low bandwidth force-control, thereby restricting the bandwidth of the elasticity control.        Antagonistic-controlled stiffness. The resultant elasticity of the joint is controlled by means of two springs being tensioned from two directions, similarly to human biceps or triceps. It is a great disadvantage of this solution that for variable resultant elasticity the insertion of springs having non-linear elongation-force characteristics is required, thereby the precise mechanical realization of which makes this solution rather complicated.        Structure-controlled stiffness and mechanically controlled stiffness. These are mechanically complicated solutions and hard to realize particularly in small size.        
A modern robotic joint or actuator with variable stiffness all the more can fulfill the expectations, the more and the higher efficiency from the following features can be met:                Backdrivable: The joint is capable to react to impacts, that is, for example, upon external force it can turn in an appropriate degree. Backdrivability is characterized by the percentage of useful torque of the robotic arm, by which it can be rotated back to be backdriven. E.g. if a robotic arm is able to exert a torque of 1 Nm and can be backdriven by a torque of 0.3 Nm, then 30% of the useful torque is required to be exerted for backdriving it, therefore the respective backdrivability of this exemplary robotic arm is 70%.        Suitable for high bandwidth force-control: it is capable to meet demands upon dynamic movement, and having high reaction speed control system.        Capable of variable elastic behavior: suitable for realizing the variation of at least linear spring constant.        Having good mass-force-consumption ratio: low mass, simple mechanical structure, which, in relation to itself can exert an appropriate force, whilst, having low consumption. Keeping consumption at a low rate is of particular significance with mobile robots.        
The robotic joint according to the study of A. Albu-Schaffer, C. Ott, U. Frese, and G. Hirzinger, Cartesian impedance control of redundant robots: Recent results with the DLR light-weight-arms, Proc. IEEE Int. Conf. Robotics and Automation (ICRA '03), Vol. 3, pp. 3704-3709 (2003) meets only partly the above features to a satisfactory degree. The joint according to the study, exhibits a limited elasticity only and uses a torque gauge sensor. Robotic joints are disclosed in another study by A. Albu-Schäffer, O. Eiberger, M. Grebenstein, S. Haddadin, C. Ott, T. Wimböck, S. Wolf és G. Hirzinger, Soft Robotics: From Torque Feedback-Controlled Lightweight Robots to Intrinsically Compliant Systems, IEEE Robotics & Automation Magazine, Vol. 15, No. 3, pp. 20-30 (2008), as well.
A decisive majority of known robotic joints are not originally backdrivable. The reason behind this is the fact that the various known robotic joints have in most cases an electronic drive mechanism using so-called BLDC (brushless direct current) motors. It is a great disadvantage of the application of BLDC motors that they are designed for a speed significantly higher than the speed generally exerted by a joint. In case of a driving mechanism using BLDC motor, therefore, a transmission of high—many tenfold or even up to hundred-fold—transmission ratio is to be used. The use of high transmission ratio, in many cases causes high frictional loss and increases the moment of inertia of the motor; consequently the robotic joint has a relatively low backdrivability. Partly due to this, the thereby obtained force regulation generated by active control cannot have high bandwidth because of mechanical limitations, accordingly is incapable of manipulating e.g. sudden impact change-of-force.
The study of J. W. Hurst, J. E. Chestnutt, and A. A. Rizzi, An actuator with mechanically adjustable series compliance, tech. report CMU-RI-TR-04-24, Robotics Institute, Carnegie Mellon University, April, 2004, discloses a mechanically complex structure for effecting variable elastic behavior, which does not meet any of the above criteria. The same disadvantages characterizing the solution disclosed in WO 2008/015460 A2.
According to the study of D. A. Lawrence, L. Y. Pao, A. C. White, and W. Xu, Low cost actuator and sensor for high-fidelity haptic interfaces, 12th International Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2004, pp. 74-81, a stepper motor is used in a haptic interface. The torque-regulated control of the stepper motor is disclosed in the study. It is furthermore explained in the study that because of the use of stepper motors, an inexpensive actuator can be developed having low friction, no backlash and being ready to exert high torque. It is furthermore explained that use of a transmission is undesirable because of the caused increased moment of inertia.
In robotics, stepper motors are known to be used for positioning only. Such applications are disclosed in the following documents.
In U.S. Pat. No. 4,618,808, U.S. Pat. No. 5,231,342, US 2002/0039012 A1 robotic applications are disclosed, wherein stepper motors are used for conventional positioning purposes.
In U.S. Pat. No. 5,426,722 stepper motors are used also in stepping mode; a robot of multiple degrees of freedom is controlled with stepper motors, with no feedback. In this solution, the individual joints are rotated in discrete steps, and the speed of the turns is controlled by the frequency of stepping.
In U.S. Pat. No. 5,760,503 miniaturizable stepper motors are disclosed. According to the document, the stepper motors are used to directly drive robot arms by means of exploiting the positioning capabilities of the stepper motors.
In US 2010/0234967 A1 it is mentioned that stepper motors could be used in robotic applications, but it is emphasized at the same time that the aforementioned direct current motors are widely used in robotic actuators.
In the study of M. Bodson, J. N. Chiasson, R. T. Novotnak and R. B. Rekowski, High-Performance Nonlinear Feedback Control of a Permanent Magnet Stepper Motor, IEEE Transactions on Control Systems Technology, Vol. 1, No. 1, pp. 5-14 (1993) an exemplary solution for linearizing stepper motors is disclosed.
In view of the known solutions the need has arisen to provide a driving mechanism applicable preferably as a robotic joint or actuator, which is capable of exerting a pre-determined torque characteristic. The use of a driving mechanism exerting a pre-determined torque characteristic will enable emulation of movements regulated by various scientific laws.